We took Middle Cat to the vet earlier in the week, just for a basic checkup (she hadn't been in a while, and she IS seventeen). The bloodwork came back today, and they said that there are very mild changes in kidney function, so they recommend putting her on cat food designed for management of renal disease.
The thing is, in the past Middle Cat has had some serious food allergies (they began after I gave her some cat food designed to prevent hairballs). What we were told at the time was that cats with this kind of irritable bowel problem are reacting to the proteins in cat food, which are overwhelmingly from chicken, so one solution is to give them food made out of more unusual proteins, which they haven't yet developed a reaction to. So for a time, Middle Cat was eating food made from duck, rabbit, and venison. (Which I don't think she appreciated as much as she should have!) Also, so many pet foods are bulked up with corn, and cats have a hard time digesting corn, that we were advised to avoid it as well (so she was eating exotic meats with rice).
Over the years, she's had no problem being transitioned back to non-exotic meat foods, so now she eats stuff made from chicken, beef, turkey, and fish with no problem (in fact, she looooooves tuna, which often gets put in cheap cat food, and is so strongly flavored it can make cats unwilling to eat anything else).
But the biggest thing is that we've ALWAYS bought really good quality food. Stuff made with human-grade ingredients, without corn, and WITHOUT BY-PRODUCTS. For instance, these are the ingredients in the food we currently give her: Chicken, Chicken Liver, Turkey, Chicken Broth, Carrots, Natural Chicken Flavor, Sweet Potatoes, Squash, Zucchini, Cranberries, Blueberries, Guar Gum, Dicalcium Phosphate, Carrageenan, Ground Flaxseed, Potassium Chloride, Calcium Carbonate, Taurine, Iron Proteinate (a source of Chelated Iron), Beta-Carotene, Zinc Proteinate (a source of Chelated Zinc), Vitamin E Supplement, Choline Chloride, Cobalt Proteinate (a source of Chelated Cobalt), Thiamine Mononitrate, Copper Proteinate (a source of Chelated Copper), Folic Acid, Manganese Proteinate (a source of Chelated Manganese), Niacin, d-Calcium Pantothenate, Sodium Selenite, Vitamin D-3 Supplement, Pyridoxine Hydrochloride, Riboflavin Supplement, Vitamin A Supplement, Vitamin B-12 Supplement, Potassium Iodide, Biotin. And I have had really healthy, long-lived cats. NLLDH swears that Youngest Cat (who was feline leukemia-positive) lived as long and as healthily as he did because he got such good food.
So now I've been looking these renal diet foods up online. They ALL have meat by-products -- ALL of them! And not just as a component -- in some cases, as the first ingredient listed after water! Now, a few of them name specific by-products (e.g. "pork by-products"), which is supposed to be better than the generic "meat by-products," but I really dislike the idea of feeding her by-products, given what can be in that crap. IAMS is also chock full of corn meal -- yuck. (Hill's Science Diet has some cornstarch, but I don't know if that's quite as bad, and it's not one of the very first ingredients, so...)
It's surprising to me how much this bothers me, but it does. I don't WANT to give her messed-with by-product crap -- I want to give her the good stuff we've always given her. But I guess we'll get some of the prescription stuff from the vet and see how it goes. Blechh.